The present invention relates generally to charging devices and charging insulating layers. In particular, the present invention is directed to a contact charging brush and method of charging an insulating layer with said brush.
The present invention is directed to contact brush charging, brush apparatus and method of use wherein the individual fibers, upon contacting the insulating surface to be charged, do not electrically short out or otherwise destructively interfer with the electrical properties of the insulating layer as a result of certain imperfections in the layer.
In an electrostatographic reproducing apparatus commonly used today, a photoconductive insulating member may be charged to a negative potential, thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostaic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder referred to in the art as toner. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive area. This image may be subsequently transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to the support surface the photoconductive insulating surface may be discharged and cleaned of residual toner to prepare for the next imaging cycle.
In such electrostatographic apparatus in use today various types of charging devices have been used to charge the photoconductive insulating layer. In commercial use, for example, are various types of corona generating devices to which a high voltage 5,000 to 8,000 volts may be applied to the corotron device thereby producing a corona spray which imparts electrostatic charge to the surface of the photoreceptor. In addition, the corona spray generates ozone and other species which have to be collected or neutralized. Alternatively, the photoconductive insulating layer may be charged with a brush charging device which is brought into contact with the photoconductive insulating layer and to which a potential of the order of 1,200 volts is applied during the charging process. This provides a constant voltage charging process wherein, for example, if a thousand volts is applied to the brush the insulating layer maybe charged to 800 volts. This is in contrast to the corona generating devices which are based on a constant current process and therefore have large fluctation in the potential eventually developed on a conductive insulating layer. In brush charging there is a dramatic reduction in ozone generation compared to corona charging despite the fact that there may be a small amount of corona. Brush charging is 100% efficient in terms of the current going to the photoreceptor whereas corotron charging is only about 10% efficient in terms of the current going to the photoreceptor. Generally, contact brush charging has the advantage of being relatively insensitive to process speed or photoconductive insulating layer electrical history within normal operating range. In other words, in contact brush charging devices if in one cycle the photoreceptor is charged and subsequently discharged by exposure to the light and shadow pattern to provide varying potential levels, upon subsequent charging with a contact brush charging for making subsequent copies the photoreceptor will be recharged only to the initial uniform potential.
While contact brush charging provides these advantages, it also suffers from serious deficiencies in that the individual brush fibers are typically made from electrically conductive materials which may upon contact with imperfect areas of the insulating surface result in shorting of portions of the insulating surface giving rise to deletions in the output copy. In the commercial manufacture of photoreceptors (photoconductive insulating layers) it is very difficult to produce a whole layer whether it be a drum or belt wherein the dielectric strength of the layer is precisely uniform throughout the entire layer. It often happens as a result of a manufacturing defect, or contamination, or other matters that are not fully understood, that certain imperfections exist in these photoreceptors. In particular, they may have areas of relatively low dielectric strength which may be visible or invisible to the unaided eye. These defects usually are within the photoreceptor layer and randomly distributed throughout the layer. When such photoreceptors are used in commercial embodiments with contact charging apparatus employing conductive fibers the fibers contact the photoreceptor and in those areas of imperfection in the photoreceptor a path where high current densities can flow produces electrical shorting to the conductive backing on the photoreceptor layer. This shorting can produce localized heating, melting, and oxidation of the polymeric materials followed by out gassing of vaporized polymeric causing devastating irreversible damage. The result is a mechanical flaw in the photoreceptor which is either a crater or a hole in the photoreceptor layer. While initially this flaw may have been an invisible defect, it now becomes visible appearing as a hole of 1 to 2 mm in the photoreceptor surface. This "pinhole" shows up as a copy quality defect since it can act as a mechanical toner trap during the development cycle and can develop out as a black spot. It can also appear as an undeveloped area, as a result of this particular small portion no longer behaving as a photoconductive insulating layer.
Photoreceptors exhibiting this "pinhole" effect can be in a variety of different configurations including plates, drums, flexible belts and the like. Typical photoreceptors include one or more photoconductive layers on a supporting substrate. The supporting substrate may be conductive or it may be coated with a conductive layer over which photoconductive layers may be coated. Alternatively, the multilayered electrophotoconductive imaging photoreceptor may comprise at least two electrically operative layers, a photogenerating or charge generating layer and a charge transport layer which are typically applied to the conductive layer. For further details of such a layer, attention is directed to U.S. Pat. No. 4,265,990 the disclosure of which is hereby herein incorporated by reference in its entirety. In all these varying structures several of the layers may be applied through vacuum deposition techniques for very thin layers. During the several fabrication processes one or more of the layers may be exposed to contamination by foreign matter (dust) or experience other process deviations. Small imperfections give rise to "pinhole" effects with which the present invention is concerned.